Maintaining the accurate output of end products of a process can require the monitoring of output and modification of the process to correct unwanted characteristics in the end product. Systems for monitoring and modifying a process are collectively known as control systems. Control systems require appropriate accuracy and reliability to properly monitor and modify a process.
Control systems typically include controllers, measurement instruments and control elements. The basic elements of a control system are shown in FIG. 1. Controllers perform the computational and/or logical functions that determine if the process output is as desired. Measurement instruments are used to monitor various aspects of a process such as flow rate, temperature, or pressure. Control elements such as valves or dampers are used to modify a physical aspect of a process such as liquid flow or air flow.
In the control system as shown in FIG. 1, a signal representing the desired process output is input into the controller from a user. A signal representing the actual process output, as measured by measurement instruments, is also input into the controller. The controller generates one or more control signal(s) and sends the control signal(s) to the control element which modifies the process so that the actual process output matches the desired process output. The control signals change with time as a result of the controller""s reaction to changes in desired process output; variations in the actual process output; process irregularities which should not cause control element reaction; imperfections in system instrumentation; signal error resulting from environmental conditions; and the inability of the control element to exactly match the intended physical adjustment to the process.
The ability of a control element to create an accurate physical adjustment is limited by the characteristics of the control element. When control systems attempt to attain accuracy beyond the inherent capabilities of the control element, oscillations and system instability can result. Oscillation in a control system causes increased movement of the control element which causes excessive wear and premature failure of the control system. Oscillation, wear, and failure tend to reduce the accuracy of the process, and therefore negatively impact the actual process output.
Continuous signal variations to the control element can cause the control element to undergo physical adjustment much more often than desirable, and much more often than necessary to control the process within acceptable limits. An increased number of physical adjustments can result in excessive wear. Excessive wear in the control element reduces the accuracy and reliability of the control element and thus the accuracy and reliability of the control system.
Prior attempts to reduce the reactions of a control element to insignificant changes in the input control signals include: reducing signal transmission errors and uncontrollable process jitter by applying a filter; reducing inaccuracies of measurement instruments, control elements or other interposed equipment by using high accuracy equipment; or reducing unwanted small changes by applying a dead band to the inputs of the control element.
Signal transmission errors and jitter, often collectively called xe2x80x9cnoisexe2x80x9d, can be reduced but not eliminated through the use of filtering of control system signals. Applying signal filtering can, however, result in a delay in the control system""s response, and can cause a control system to become unstable. Additionally, the remnants of noise which can exist in a control system using filtering can still result in unwanted movement of the control element.
Other attempts to reduce unnecessary control element movement have largely depended on the type of signal applied to the control element and the type of control element. Systems using pneumatic-powered control elements, based on using air pressure in conjunction with electrical control, employ different schemes for coordinating the electricity and the air pressure. Such systems typically employ mechanical friction and/or electrical techniques to create a dead band. Dead band, as used herein, refers to a range of signal values where changes of an input control signal falling within the range do not cause a response in the control system output. Dead band can be used to reduce unwanted control element movement in pneumatic systems, however, use of dead band reduces the accuracy of the control element. This reduced accuracy tends to degrade the actual process output, which can result in significant reactions in the control system, which can result in increased unwanted movements of the control element.
Systems which create dead band using friction are typically inaccurate since friction can change with age, temperature, humidity, vibration of mechanical elements, changes in the characteristics of the process or other environmental factors. Variability of the dead band reduces the predictability of the control element. If the control element is not predictable, controlling the process is extremely difficult. Typically, the dead band in systems affected by environmental factors is significantly increased during aging, resulting in the inability of the control element to respond to desired small changes in the process. This inability of the control element to respond to desired changes results in reduced process accuracy and efficiency.
In systems with electrical dead band, mechanical friction may act to hinder the electrical dead band or to enhance the electrical dead band. Systems using electric-powered control elements typically try to minimize all sources of friction, and rely solely on electrical dead band. In systems where friction has been reduced to negligible levels, system dead band is created using various electrical circuit techniques. Although the electrical circuits of such systems can have increased accuracy and reduced failure due to wear, noise can still cause system inaccuracy and the mechanical control elements of the system can still suffer from wear. The dead band of such systems can be set to allow for the inaccuracies and wear characteristics of the mechanical system elements. However, if dead band is reduced beyond the inherent accuracy of the mechanical elements, the control element can become unstable, resulting in wear and process disturbances. If the dead band is large, process accuracy and efficiency are reduced.
In systems using improved measurement equipment, control systems, and electrical communication techniques to remove noise from the control system, dead band can be reduced. However, when noise is removed from a control system, control element stability becomes a limiting factor for how small the dead band can be. For systems using a single dead band range chosen for both a particular system""s demand characteristics and to prevent control element instability, the control elements are prevented from providing optimal control.
Accordingly, it is desirable to have a control device which can reduce the size of the dead band to increase accuracy, and still preserve dead band to prevent insignificant movements of a control element substantially improving the overall accuracy of the control system.
By associating different dead bands with different signal changes or categories of signal change, the dead band can be substantially optimized to match the noise and accuracy characteristics of the signals of a control system. A reduced or minimum dead band can therefore be used for each signal. This allows for a system that increases or maximizes overall accuracy and reduces or minimizes control element wear.
An improved control device and method is provided for controlling a control element by receiving a controller demand signal and a control element sensor signal, and processing the controller demand signal and the control element sensor signal to determine if there is a change in the controller demand signal and/or a change in the control element sensor signal. A first dead band range and/or a second dead band range is determined based on the change in the controller demand signal and/or based on the change in the control element sensor signal. An error signal is calculated from the controller demand signal and the control element sensor signal. A determination is made as to whether the error signal is outside of the first dead band range and/or the second dead band range if the controller demand signal is changing and/or whether the error signal is outside of the second dead band range if the controller demand signal is not changing. A control signal for controlling the control element is generated based on the determination of the dead band ranges and whether the calculated error signal is outside of the first and/or second dead band ranges.